1. Field of the Invention
The present invention relates to a process for partial shaping of flat glass or comparable glass-ceramic articles. It also relates to an apparatus for performing that process.
2. Description of the Related Art
A pre-shaped flat glass article, which comes in the form of a table or in the form of an endless sheet, and/or a correspondingly formed glass-ceramic article, frequently must be partially shaped for certain application purposes. This shaping includes, for example, the formation of a turned up or raised edge region around a hole, the flanging of an outer edge or the formation of depressed or raised regions in glassy table tops, glass doors or glass-ceramic cook tops.
In practice different shaping methods have been used for this purpose. The shaping techniques according to the prior art include sinking under its own weight, pressing, imprinting or vacuum drawing.
A substantial disadvantage of the method of sinking under its own weight is the limited degree of shaping that can be obtained. Only limited forms can be obtained, especially sharp edged shapes or narrow radii.
The main disadvantage of the pressing and imprinting methods is that a forming material or tool must come into contact with the surface of the glass or the glass-ceramic part so that there is an inherent quality loss at the surface.
The disadvantage of vacuum drawing is the necessity of positive contact with the edge region or rim of the glass or glass-ceramic part around the shaping region, so that no vacuum loss arises at the onset of the shaping process. Usually the process is exclusively used for drawing softened glass and/or glass-ceramic masses into cavities of a mold that is planar at the edges.
A process and associated apparatus for making glass bodies for electronic tubes, especially display screen parts, are disclosed in DE 25 28 421 A1. In this process a substantially planar glass plate is received in a holding or clamping unit at its rim or edge region as a starting product (semi-finished product). It is then heated over the softening point of the glass in a burner and then shaped by means of a suitably formed die or press member. The holding or clamping unit is generally formed so that overpressure or under pressure can be produced in the space under the glass plate, whereby the shaping process of the glass plate can also be influenced. In case of an under pressure an actual vacuum drawing method is described.
These known methods however cannot be used for the partial shaping of flat glass and/or comparable glass-ceramic articles or parts.